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FER  11  1914 


ELEMENTARY 
ELECTRICAL  TESTING 


MONOGRAPH  2 

JOINT    COMMITTEE   SERIES 

NATIONAL    EDUCATION'  ASSOCIATION    EDITION 


X 


CONTRIBUTED  BY  THE  TECHNICAL  STAFF  OF  THE 

WESTON  ELECTRICAL  INSTRUMENT  CO. 
NEWARK,  N.J. 


THE  JOINT  COMMITTEE  MONOGRAPH  SERIES 


The  following  is  a  list  of  Monographs  written  or  being 
written  by  the  technical  staff  of  the  manufacturers  mentioned, 
and  issued  in  co-operation  with  the  Joint  Committee  on  Physics. 
These  Monographs  are  intended  to  convey  to  teachers  the 
point  of  view  of  men  of  affairs  as  to  the  principles  and  facts 
worth  teaching  to  high  school  students  in  each  specialty. 

1.  Announcement. 

2.  Elementary  Electrical  Testing. 

Weston  Electrical  Instrument  Company,  Newark,  N.  J. 

3.  Edison  Storage  Batteries. 

The  Edison  Storage  Battery  Company,  Orange,  N.  J. 

4.  Hydraulic  Machinery. 

Gould  Pump  Company,  Seneca  Falls,  New  York. 

5.  Mechanics  of  the  Sewing  Machine. 

Singer  Sewing  Machine  Co.,  Singer  Building,  New  York  City. 
Others  are  projected. 


ELEMENTARY 
ELECTEICAL   TESTING 


MONOGRAPH  2 
JOINT  COMMITTEE   SERIES 

NATIONAL  EDUCATION  ASSOCIATION  EDITION 


CONTRIBUTED   BY   THE   TECHNICAL   STAFF   OF   THE 

WESTON  ELECTRICAL  INSTRUMENT  CO, 
« * 

NEWARK,    N.  J. 


COPYRIGHT,    1914 

BY 
WESTON  ELECTRICAL  INSTRUMENT  CO. 


ARGUMENT 


THIS  monograph  is  prepared  as  evidence  of  our  belief  in 
the  new  movement  to  make  education  more  practical. 
The  Weston  Electrical  Instrument  Company  is 
convinced  that  the  public  high  school  instructors  desire  to  use 
pedagogic  material  drawn  from  real  life  and  prefer  to  perform 
laboratory  experiments  which  teach  the  fundamentals  of  science 
as  far  as  possible  with  commercial  equipment,  such  as  the  student 
will  later  use  or  see  used,  and  with  which  he  will  later  be  expected 
to  be  familiar. 

Assured  that  teachers  want  to  know  what  technical  men 
in  the  industries  believe  to  be  fundamental,  we  have  prepared 
brief  suggestions  relating  to  the  preparation  of  a  course  in 
electrical  measurements.  Representatives  of  our  house  have 
been  observing  the  work  of  schools  for  several  years.  In  issuing 
this  monograph,  we  have  consulted  authorities  on  pedagogics. 
We  have  also  received  the  advice  of  school  men  and  have  then 
printed  such  matter  as  appealed  to  our  judgment. 

Some  of  the  suggestions  offered  may  be  beyond  the  present 
scope  of  high  school  physics,  although  extremely  elementary 
from  our  standpoint. 

Since  this  is  neither  presented  as  a  text  book,  nor  offered 
as  a  complete  outline  of  a  course  of  study,  but  merely  contains 
material  offered  for  consideration,  it  is  probable  that,  as  this 
movement  progresses,  exercises  which  are  at  present  too  advanced 
may  later  be  acceptable  and  even  necessary. 

Exercises  credited  to  authors  of  text  books  have  in  some 
instances  been  abbreviated  but  not  altered  in  detail,  even  though 
in  our  judgment  and  in  the  judgment  of  members  of  the  Joint 
Committee  changes  could  well  be  made.  We  have,  however, 

280152 


4-  ARGUMENT 

appended    our    opinions    when   we    differed    considerably    with 
the  author,  or  believed  suggestions  were  appropriate. 

We  hereby  express  our  thanks  to  the  authors  and  publishers 
who  permitted  us  to  quote  from  their  works. 

We  wish  to  acknowledge  our  indebtedness  to  the  members 
of  the  Joint  Committee  on  Physics  who  have  advised  us  in 
both  the  general  plan  of  the  monograph  and  in  the  selection 
and  arrangement  of  the  material.  All  of  our  suggestions  which 
were  adjudged  by  the  committee  members  to  be  impracticable 
under  existing  conditions  pertaining  to  schools  have  been 
eliminated.  However,  every  effort  has  been  made  to  make 
this  monograph  represent  the  judgment  of  our  Staff,  rather 
than  that  of  the  educators  consulted. 

WESTON  ELECTRICAL  INSTRUMENT  COMPANY. 


CONTENTS 


SUBJECTS 

PACK 

Argument '. 3 

The  Extensive  Use  of  Electrical  Energy 7 

Current 8 

Connections  for  Voltmeters  and  Ammeters 13 

Power 21 

Resistance 23 

Insulation 25 

The  Weston  Direct-current  Ammeter 33 

The  Weston  Direct-current  Voltmeter .  .  37 


EXPERIMENTS 

I.  Resistance  and  Current  in  a  Divided  Circuit 8 

II.  The  Measurement  of  the  Current,  Voltage,  and  Power  of  a  Model 

Lighting  Circuit.     Two-wire  System 10 

III.  The  Fall  of  Potential  along  a  Conductor 13 

IV.  Fall  of  Potential  in  a  Lamp  Bank 15 

V.  The  Number  of  Watts  for  one  B.T.U.  per  Second 17 

VI.  Heating  Effect  of  an  Electric  Current 18 

VII.  Efficiency  Test  of  an  Electric  Motor 21 

VIII.  The  Determination  of  Low    Resistances  by  the  Ammeter  and 

Voltmeter  Method 23 

IX.  The  Determination  of  High  Resistances  by  the  Voltmeter  Deflec- 
tion Method 25 

X.  The  Wheatstone  Bridge 27 

XL  The  Slide  Wire  Bridge 30 

XII.  The  Effect  of  Temperature  on  the  Resistance  of  a  Lamp  Filament  31 

XIII.  The  Study  of  an  Ammeter 35 

XIV.  The  Study  of  a  Voltmeter 39 

XV.  The  Action  of  a  Simple  Cell 40 

5 


ELEMENTARY   ELECTRICAL  TESTING 


THE  EXTENSIVE  USE  OF  ELECTRICAL  ENERGY  IN 
NUMEROUS   INDUSTRIES 

In  re-outlining  the  course  of  study  in  high  schools,  due  con- 
sideration should  be  given  to  the  fact  that  changes  in  the  relative 
importance  of  certain  industries  have  recently  taken  place,  and 
that  these  changes  affect  both  the  home  and  the  civic  life  of  our 
people  in  various  communities. 

The  new  census  gives  figures  to  support  the  statement  that 
electrical  industries  are  economically  more  important  than  the 
chemical  industries  in  point  of  the  number  of  wage-earners 
employed  and  in  the  value  of  the  product  manufactured  each 
year. 

It  is  estimated  that  ten  billion  dollars  are  now  invested  in 
electrical  industries.  About  one-fourth  of  this  represents  the 
capital  invested  in  power  developments  of  a  semi-public  nature. 

The  educational  significance  of  these  facts  is  two-fold. : 

First.  In  emphasizing  that  more  attention  must  be  given  by 
instructors  to  the  industrial  applications  of  electricity,  and  to 
physics  generally. 

Second.  In  the  direct  answer  which  they  give  to  the  question 
as  to  what  constitutes  practical  ideas  from  the  business  man's 
point  of  view.  We  will  elaborate  this  point.  The  greater  por- 
tion of  this  tremendous  investment  is  dependent  upon  the  main- 
tenance of  circuits  having  a  pressure  of  110  volts  or  over.  Accord- 
ingly the  first  things  that  students  should  learn  are  the  facts  con- 
cerning these  circuits.  For  developing  means  of  teaching  these 
and  other  facts  the  country  is  dependent  upon  its  educators. 
It  is  from  teachers  only  that  the  student  may  obtain  comprehen- 

7 


8  ELEMENTARY   ELECTRICAL  TESTING 

sive  ideas  relating  to  the  materials,  arrangement  and  methods 
of  distribution  of  electrical  circuits.  Just  as  the  water  and  gas 
mains,  with  their  innumerable  outlets  and  ramifications,  supply 
a  city  with  two  necessities,  the  electrical  conductors  give  us 
access  to  an  agent — a  source  of  energy — which  has  become 
more  necessary  to  us  than  gas,  and  in  frequent  instances  is 
almost  as  indispensable  as  water.  And  this  agent,  this  source 
of  energy,  is  an  electric  current. 

It  seems  logical  and  proper,  therefore,  that  a  course  in  ele- 
mentary electrical  measurements  should  begin  with  the  con- 
sideration of  the  subject  from  the  standpoint  of  current. 

CURRENT 

The  following  exercise,  published  in  the  Manual  of  Fuller 
and  Brovvnlee,*  Experiment  77,  is  here  given  in  part  as  an  illus- 
tration of  a  method  of  teaching  current  distribution. 


EXPERIMENT   I 
RESISTANCE  AND   CURRENT   IN  A  DIVIDED  CIRCUIT 

Object.  To  compare  (a)  the  currents  in  the  branches  of 
a  divided  circuit  with  the  resistance  of  those  branches;  (6) 
the  total  resistance  with  the  resistance  of  the  branches. 

Apparatus.  Lamp  board  like  that  shown  in  Fig.  1;  32- 
candle-power  lamps  to  fill  board;  3  ammeters;  voltmeter,  with 
connecting  wires;  connections  to  110  volts  B.C.  circuit. 

Experimental.  Proper  connections  for  a  circuit  of  two 
branches,  like  that  shown  in  Fig.  1,  are  to  be  made.  The  resist- 
ance in  each  branch  of  the  circuit  consists  of  an  equal  number 
of  similar  incandescent  lamps,  connected  in  parallel.  The 
ammeters  are  so  connected  that  the  total  current  through  both 
branches  can  be  read  and  also  the  individual  current  in  each 
branch.  The  terminals  of  a  voltmeter,  which  is  not  shown, 
are  to  be  connected  to  the  terminals  of  any  portion  of  the  cir- 
cuit whose  resistance  is  desired. 

*  Published  by  Allyn  &  Bacon,  Boston. 


RESISTANCE  AND  CURRENT  IN  A  DIVIDED  CIRCUIT       9 


All  the  lamps  on  both  sides  are  to  be  turned  on  and  reading 
of  each  ammeter  recorded.  The  voltmeter  is  then  connected 
in  succession  to  the  terminals  of  each  branch  circuit  and  to  the 
terminals  of  the  combined  circuit  and  the  readings  obtained 
recorded  in  tabular  form  near  the  top  of  the  left-hand  page. 
All  the  lamps  but  one  on  one  branch  are  then  turned  out,  leav- 
ing all  the  lamps  in  the  other  branch  of  the  circuit  burning. 
Readings  of  the  voltmeter  and  ammeters  are  taken  and  recorded 


QOQ 


i 


FIG.  1. — Lamp  Board,  Ammeters  and  Connections. 

as  before.  Make  the  following  additional  combinations  in  the 
two  branches  and  record  the  results:  2  lamps  and  3  lamps;  2 
lamps  and  4  lamps;  2  lamps  and  5  lamps. 

A  simple  diagram  of  connections  should  be  made,  and  a  brief 
description  of  the  method  of  making  the  tests  should  be  given. 
From  the  readings  of  the  instruments  the  resistance  of  each 
branch  and  the  resistance  of  the  entire  circuit  should  be  calculated 
for  each  case  by  the  application  of  Ohm's  Law. 

Discussion.  Does  increasing  the  number  of  lamps  in  parallel 
in  a  circuit  increase  or  decrease  the  resistance  of  the  circuit? 


10  ELEMENTARY  ELECTRICAL  TESTING 

When  the  number  of  equal  known  resistances  are  connected  in 
parallel,  give  a  rule  for  finding  the  combined  resistance. 


EXPERIMENT  II 

THE  MEASUREMENT  OF  THE  CURRENT,  VOLTAGE,  AND 
POWER  OF  A  MODEL  LIGHTING  CIRCUIT.  TWO-WIRE 
SYSTEM 

This  experiment  is  from  Wiley's  Loose  Leaf  Manual.*  It 
is  more  elaborate  but  attains  the  same  end  as  the  preceding  one. 

Apparatus  in  the  Laboratory:  Model  lighting  circuit  board; 
110-volt  direct  current. 

Apparatus  from  the  Stock  Room:  Ammeter;  voltmeter; 
yard  stick;  15-ampere  fuses. 

The  line  wires  are  No.  14  German  silver,  180  ohms  per  mil- 
foot,  to  furnish  in  the  limited  space  of  the  laboratory  conditions 
similar  to  those  of  an  actual  incandescent  lighting  circuit  in  a 
three-story  building.  (See  Fig.  2.) 

All  readings  of  current,  voltage,  etc.,  should  be  recorded  on  a 
single  outline  diagram  of  the  line. 

The  binding  posts  at  the  end  of  the  line  are  to  be  regarded 
as  the  source  of  power.  Measure  the  voltage  at  these  terminals 
and  the  current  in  each  section  of  the  line  at  this  voltage. 

Measure  the  distance  from  the  terminals  to  the  first  group  of 
lamps,  also  the  distance  from  each  group  to  the  next  succeeding 
group.  (Measure  in  each  case  to  the  point  on  the  mains  at  which 
the  short  leads  from  the  lamps  are  attached.  The  resistance 
of  the  short  copper  leads  to  the  lamps  may  be  neglected.) 

From  its  length  and  mil-foot  resistance,  compute  the  resist- 
ance of  each  section  of  the  line.  Compute  also  the  "  drop  " 
over  each  section  and  the  voltage  at  each  group  of  lamps.  Check 
your  computed  voltage  by  comparison  with  the  voltmeter  read- 
ings across  the  lamps. 

Since  the  voltage  may  fluctuate,  one  voltmeter  should  be  kept 
connected  across  the  supply  terminals,  and  the  voltage  at  the 
lamps  should  be  read  from  a  second  voltmeter  when  the  voltage 
at  the  terminals  is  at  the  proper  value. 

*  Published  by  John  Wiley  &  Sons,  Inc.,  New  York. 


THE  MEASUREMENT  OF  THE  CURRENT 


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12 


ELEMENTARY  ELECTRICAL  TESTING 


From  your  data  determine  also: 

1.  The  watts  delivered  at  the  binding  post  terminals. 

2.  The  watts  expended  in  each  section  of  the  feed  wire. 

3.  The  watts  expended  in  each  group  of  lamps. 

4.  The  total  watts  supplied  to  the  lamps  and  the  total  line 

loss. 

5.  The  "  efficiency  "  of  the  line,  i.e.,  the  percentage  of  the 

total  power  supplied  which  is  delivered  to  the  lamps. 

6.  The  resistance  of  each  group  of  lamps,  and  the  average 

hot  resistance  of  each  lamp. 


FIG.  3. — Connection  for  Ammeter  with  Self-contained  Shunt. 


FIG.  4. — Connection  for  Ammeter  with  Detachable  Shunt. 


If  conditions  permit,  we  would  suggest  that  a  teacher  would 
better  have  small  groups  of  students  use  a  part  of  the  actual 
lighting  circuit  in  the  school,  in  place  of  a  model,  and  follow  the 
directions  given  in  the  experiments  as  far  as  possible. 

This  plan  is  suggested  by  the  letter  on  page  22  of  our  Mono- 
graph B-l,  written  by  Mr.  C.  W.  Parmenter,  Head  Master  of 
the  Mechanics  Arts  High  School,  Boston.  The  plan  is  also  used 
by  Prof.  H.  H.  Higbie  of  the  Wentworth  Institute,  Boston,  Mass. 


THE  FALL  OF  POTENTIAL  ALONG  A  CONDUCTOR         13 

CONNECTIONS   FOR  VOLTMETERS  AND   AMMETERS 

The  student's  attention  should  be  called  to  the  fact  that 
an  ammeter  should  always  be  connected  in  series  with  the  line, 
whereas  a  voltmeter  should  be  connected  across  the  line.  See 
Figures  3,  4  and  5. 


FIG.  5. — Connection  for  Voltmeter. 


EXPERIMENT  III 
THE   FALL   OF  POTENTIAL  ALONG   A   CONDUCTOR 

The  nature  of  a  divided  circuit  and  the  current  in  it  may  have 
been  called  the  primary  idea  in  the  preceding  experiments,  but 
they  served  as  well  as  means  of  discussing  the  theory  that*it  is 
pressure  which  actually  forces  the  current  through. 

Schools  have  for  a  long  time  given  an  exercise  showing  the 
"  Fall  of  Potential  along  a  Conductor,"  and  it  merely  remains 
for  the  teacher  to  give  this  exercise  a  more  practical  atmosphere 
to  have  it  continue  to  be  one  of  the  strongest  on  the  list.* 

Apparatus  Required:  Weston  ammeter,  range  3  amperes; 
Weston  voltmeter,  range  3  volts;  1  meter  slide  wire  bridge; 
storage  cell  or  2  constant  primary  cells  in  series;  No.  18  (B.  &  S. 
gauge)  40-mil  Weston  alloy  wire;  No.  24  (B.  &.  S  gauge)  20-mil 
Weston  alloy  wire. 

A  length  of  40-mil  alloy  wire  is  stretched  over  the  meter  rod 
and  securely  clamped.  The  resistance  of  40-mil  alloy  wire  is 
approximately  0.50  ohm  per  meter. 

One  storage  cell,  with  the  ammeter  in  series,  is  connected  with 
the  stretched  wire,  an  extra  piece  about  %  meter  long  is  included 

*See  Experiment  No.  29,  Laboratory  Manual  in  Physics.  Wauchope, 
Scott,  Foresman  &  Co.,  New  York  and  Chicago. 


14 


ELEMENTARY  ELECTRICAL   TESTING 


in  the  circuit  and  its  length  is  regulated  until  the  current  flowing 
is  3.00  amperes.     See  Fig.  6. 

The  3-volt  range  of  the  voltmeter  is  also  connected  as  shown 
and  primarily  the  drop  over  the  entire  length  of  wire  is  found. 

The  resistance  of  1  meter  of  wire  is  then  determined  by  the 
•pi 

formula  R  =  —. 

It  must  be  borne  in  mind,  however,  that  this  is  riot  a  "  zero  " 
method,  and  theoretically,  at  least,  is  not  correct.     That  is  to 


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say,  since  current  is  necessarily  flowing  through  the  voltmeter 
when  in  use,  the  instrument  therefore  forms  part  of  a  divided 
circuit  by  shunting  the  resistance  wire. 

If  an  improperly  constructed  voltmeter  of  low  resistance  is 
employed,  its  introduction  will  affect  the  indications  of  the  ammeter 
by  materially  reducing  the  total  resistance  of  the  circuit,  and  its 
own  indications  will  be  erratic  and  disproportionate  when  differ- 
ent lengths  of  the  resistance  wire  are  being  tested. 

Weston  voltmeters  have  sufficiently  high  resistances  to  pre- 
vent the  irregularities  referred  to,  but  nevertheless  the  best 
method  for  determining  the  total  resistance  of  the  wire  is  to  ascer- 
tain the  drop  over  definite  lengths  of  the  wire,  such  for  instance 


THE  FALL  OF  POTENTIAL  IN  A  LAMP  BANK  15 

as  10  centimeters,  tabulate  the  results,  obtain  a  mean  value  and 
determine  the  total  resistance  by  direct  proportion. 

When  sufficient  data  have  been  secured  relating  to  the  40- 
mil  wire,  a  piece  of  20-mil  is  substituted,  and  the  test  repeated. 
A  current  of  about  1.00  ampere  may  be  used.  The  resistance 
of  20-mil  alloy  is  about  2  ohms  per  meter. 

A  length  of  copper  wire  may  also  be  tested,  preferably  one 
having  a  diameter  of  0.020  inch,  and  the  relative  resistance 
of  copper  and  Weston  alloy  wire  determined. 

Caution:  Weston  alloy  wire  has  about  23.5  times  the  resist- 
ance of  copper  wire,  and  therefore  the  experimenter  must  be 
cautioned  to  add  enough  alloy  wire  to  the  circuit  to  compensate 
for  this  difference,  before  the  current  is  completed  through  the 
copper  wire. 

The  fall  of  potential  along  a  conductor  may  also  be  beauti- 
fully shown  with  this  apparatus  by  substituting  a  wire  the 
resistance  of  which  is  not  uniform.  Such  a  wire  may  easily  be 
prepared  by  scraping  or  filing  a  resistance  wire  at  irregular  dis- 
tances while  stretched,  and  by  coating  it  with  a  little  solder  here 
and  there,  afterwards  smoothing  with  emery  paper. 

The  two  voltmeter  leads  are  then  held,  one  in  each  hand, 
so  as  to  span  a  section  of  the  wire,  and  the  deflection  is  noted  and 
compared  with  the  results  obtained  with  other  sections  of  the 
same  length. 

EXPERIMENT  IV 
THE  FALL   OF   POTENTIAL   IN   A   LAMP  BANK* 

Question.   What  is  meant  by  "  Fall  of  Potential "? 

Apparatus.  110-volt  current;  three  electric  lamps;  volt- 
meter. 

Directions.  Pass  the  current  through  the  three  lamps  con- 
nected in  series  as  shown  in  the  diagram.  It  requires  electrical 
pressure  to  force  the  current  through  the  lamps.  This  pres- 
sure is  measured  by  the  voltmeter.  Connect  the  terminals 
of  the  voltmeter  to  a  and  d  to  obtain  the  volts  pressure  required 
to  drive  the  current  through  all  three  lamps.  In  like  manner 
find  the  difference  of  electrical  pressure  between  a  and  c  for  two 
lamps,  and  between  a  and  b  for  one  lamp. 

*  From  "Manual  in  Physics,"  by  Joseph  A.  Wauchope. 


16 


ELEMENTARY  ELECTRICAL  TESTING 


Results.  Volts  pressure  required  for  three  lamps  = 
Volts  pressure  required  for  two  lamps  = 
Volts  pressure  required  for  one  lamp  = 

Discussion.  Potential  is  only  another  expression  for  electrical 
pressure.  Difference  of  potential  means  difference  of  electrical 
pressure,  and  fall  of  potential  means  fall  of  electrical  pressure. 
A  voltmeter  is  used  to  measure  the  potential  difference  between 
any  two  points  in  an  electrical  circuit,  just  as  pressure  gauges  are 
used  between  any  two  points  of  a  water  pipe.  The  further  away 
from  the  pumping  station  the  greater  the  fall  of  pressure  of  the 
water  in  pounds  per  square  inch,  due  to  the  resistance  of  the  pipes. 


FIG.  7. 


D.C.  AMMETER 


So  the  further  the  electric  current  goes  in  a  conductor  the 
greater  the  fall  of  potential  in  volts,  due  to  the  electrical  resistance 
of  the  conductor.  The  greater  the  resistance  the  greater  the 
fall  of  potential.  Do  the  lamps  used  in  this  experiment  all  have 
the  same  amount  of  resistance?  Why  do  not  the  lamps  give 
light  in  this  experiment?* 

*  In  the  "Physical  Laboratory  Manual,"  by  Ball,  Hauptman  and  Bateman, 
issued  by  the  Cooper  Union  Supply  Store,  New  York,  a  different  method  is 
given,  which  consists  in  employing  a  lamp  bank,  a  gold-leaf  electroscope, 
and  a  Weston  voltmeter. 

They  call  attention  to  the  fact  that  the  leaves  of  the  electroscope  will 
diverge  less  and  less  as  the  knob  of  the  electroscope  is  connected  at  the 
points  of  decreasing  potential,  and  then  add : 

"Now  use  a  Weston  voltmeter  in  place  of  the  electroscope  and  note 
the  readings  of  the  voltmeter  for  the  same  points.  Note  how  much  more 
accurately  the  potential  difference  can  be  estimated  by  means  of  the  volt- 
meter than  by  means  of  the  electroscope. 


WATTS  FOR  ONE  BRITISH  THERMAL  UNIT 


17 


FIG.  8. 


EXPERIMENT   V 

NUMBER  OF  WATTS  FOR   ONE  BRITISH  THERMAL  UNIT 

PER   SECOND 

In  "  Physics,"  by  Mann  and  Twiss,*  the  following  exercise 

is  suggested,  which  has  long 

been     successfully     used     in 

schools    to    teach    that    the 

electrical  energy  delivered  to 

an  incandescent  light  is  trans- 

formed into  heat  energy. 

An     ordinary     16-candle- 

power,  110-volt,  electric  lamp 

is  placed  in  a  jar  containing  a 

measured    amount    of    water 

and  a  thermometer  (Fig.  8). 

The    heat    from    the     lamp 

warms  the  water.     If  the  jar 

contains  2  pounds   (1  quart) 

of  water  and   if  a  Fahrenheit   thermometer  is  used,   the  tem- 

perature of  the  water  rises  about  1J°  F.  per  minute  —  i.e., 
the  water  is  heated  at  the  rate  of  3  B.T.U. 
a  minute. 

A  voltmeter  V  and  an  ammeter  A  measure 
the  electric  power  supplied  to  the  lamp.  If 
the  voltmeter  reads  110  volts,  and  the  am- 
meter reads  J  ampere,  the  power  supplied  is 
110  (volts)  Xj  (ampere)  =55  watts.  Hence, 
roughly,  55  watts  =  3  B.T.U.  per  minute;  or 
1100  watts  =  1  B.T.U.  per  second. 

The  first  determination  of  this  relation 
was  made  more  accurately  by  the  same  Joule 
who  made  the  first  determination  of  the  rela- 
tion between  the  B.T.U.  and  the  foot-pound. 
The  apparatus  (Fig.  9)  does  not  differ  in 
principle  from  that  used  by  Joule.  A  coil 
of  platinum  wire  is  placed  in  a  jar  con- 

taining a  measured  quantity  of  water  and  a  thermometer.     A 


FIG.  9. 


Scott,  Foresman  &  Co.,  Publishers,  New  York  and  Chicago. 


18  ELEMENTARY  ELECTRICAL  TESTING 

voltmeter  V  and  an  ammeter  .A,  arranged  as  shown  in  Fig.  7, 
measure  the  number  of  watts  of  electric  power  used  in  heating 
the  coil.  The  number  of  B.T.U.  given  up  by  the  coil  to  the 
water  in  a  certain  time  is  obtained  by  multiplying  the  number  of 
pounds  of  water  in  the  jar  by  the  number  of  degrees  F.  rise  in 
temperature. 

Joule's  experiments  have  been  repeated  many  times  by  many 
other  scientists,  using  different  current  strengths  and  different 
kinds  of  coils.  The  results  of  all  of  these  experiments  show  that 
there  is  a  constant  ratio  between  the  heat  unit  and  the  unit  of 
electric  power;  and  they  give  the  accurate  value  of  this  ratio  as 

1055  watts  =  1  B.T.U.  per  second. 
4.2  watts  =  1  gram-calorie  per  second. 

The  above  exercise,  as  described,  deals  merely  with  the  rela- 
tion that  the  power  units  (watts  and  B.T.U.  per  second)  bear 
to  each  other. 


EXPERIMENT  VI 
HEATING   OF  AN   ELECTRIC   CURRENT 

The  following  directions,  reprinted  from  "  Laboratory  Exer- 
cises in  Physics,"  by  Fuller  and  Brownlee,  seem  to  be  more 
practical  than  the  preceding  one.* 

Object.  To  measure  the  number  of  calories  of  heat  fur- 
nished by  an  incandescent  lamp  and  to  calculate  the  cost. 

Apparatus:  Calorimeter;  thermometer;  16-candle-power 
incandescent  lamp;  porcelain  keyless  socket;  voltmeter;  amme- 
ter; source  of  110-volt  current;  graduate,  or  balance  and  weights; 
flexible  insulated  wire  for  connections;  watch  or  clock  with 
second  hand. 

By  allowing  a  lamp  to  heat  a  known  weight  of  water  for  a 
measured  time,  we  may  find  the  calories  per  second  furnished 
by  the  lamp.  If  we  know  the  current  and  voltage  of  the  lamp, 
we  may  estimate  the  heat  liberated  per  kilowatt-hour.  Although 
all  the  heat  liberated  by  the  lamp  will  not  be  measured  in  this 

*  See  also  Experiment  92  in  "Experimental  Physics,"  by  Smith,  Tower 
and  Furton.  Ginn  &  Co. 


HEATING  OF  AN  ELECTRIC  CURRENT 


19 


experiment,  yet  the  efficiency  of  the  lamp  as  a  heater,  as  used 
here,  compares  favorably  with  regular  heating  appratus. 

Experimental.  A  porcelain  keyless  socket  is  connected  to  a 
110-volt  line  with  an  ammeter  between  the  socket  and  the  110- 
volt  terminals  (Fig.  10).  A  voltmeter  is  connected  across  the 
terminals  of  the  socket.  A  lamp  is  then  screwed  into  the  socket 
and  the  switch  closed  in  the  circuit  to  make  sure  that  the  con- 
nections are  correct  and  that  the  instruments  read  in  the  proper 
direction.  The  lamp  is  then  turned  off  till  needed. 

Into  a  nickel-plated  brass  calorimeter  is  placed  250  grams  of 
water  at  a  temperature  six  or  seven  degrees  below  room  tem- 
perature. 


FIG.  10. 


This  is  stirred  thoroughly  with  a  thermometer  and  the  tem- 
perature noted;  immediately  the  current  is  turned  on  through 
the  lamp,  which  is  inserted  in  the  calorimeter,  the  exact  time  in 
minutes  and  seconds  being  noted. 

The  time  and  the  temperature  of  the  water  are  recorded 
in  the  tabular  form  near  the  top  of  the  left-hand  page,  the  volt- 
meter and  ammeter  also  being  noted  and  recorded.  The  lamp 
should  be  immersed  until  the  tip  rests  on  the  bottom  of  the 
calorimeter,  and  the  thermometer  should  stand  in  the  calorim- 
eter beside  the  lamp.  For  the  next  five  minutes  the  lamp 


20  ELEMENTARY  ELECTRICAL  TESTING 

burns  inverted  in  the  water.  By  moving  the  lamp  up  and  down 
in  the  water,  never  raising  it  more  than  a  quarter  of  an  inch 
from  the  bottom,  the  water  can  be  kept  stirred  and  so  of  equal 
temperature  throughout. 

The  calorimeter  should  not  be  handled  during  the  experiment. 
The  voltmeter  and  ammeter  should  be  frequently  observed,  and 
if  there  is  any  variation,  the  average  reading  for  the  whole  time 
should  be  the  one  recorded  and  used. 

When  the  lamp  has  been  in  the  water  exactly  five  minutes, 
take  it  out  promptly,  stir  the  water  vigorously  with  the  thermom- 
eter, and  read  and  record  the  temperature. 

Using  fresh  quantities  of  water,  repeat  the  test  twice.  The 
water  equivalent  of  the  calorimeter  should  be  obtained  from 
the  instructor. 

Make  a  sectional  drawing  of  the  calorimeter  with  lamp  and 
thermometer  in  place  and  with  the  connections  of  the  instrument 
shown. 

A  brief  description  of  the  method  of  the  experiment  should 
accompany  the  drawing. 

From  the  weight  of  the  water,  with  the  water  equivalent  of  the 
calorimeter  added,  and  the  change  of  temperature,  the  number  of 
calories  furnished  in  five  minutes  can  be  calculated.  The  number  of 
watt-seconds  is  found  by  multiplying  volts,  amperes,  and  seconds 
together.  From  these  two  results  calculate  the  calories  per 
watt-second  and  per  kilowatt-hour.  As  the  time  and  the  weight 
of  water  are  the  same  in  all  three  tests,  the  averages  of  tempera- 
ture changes,  volts,  and  amperes  wTill  be  used  in  the  calculation. 
The  problem  called  for  in  the  conclusion  should  be  worked  out 
in  the  note-book,  using  the  local  rate  for  electricity. 


CALCULATED  RESULTS 

Corrected  weight  of  water  (water  +  water  equivalent  of  calorimeter). ...  g. 

Average  temperature  change  in  five  minutes c. 

Calories  furnished  in  five  minutes cal. 

Calories  furnished  per  second cal. 

Watt-seconds  of  energy  used  in  five  minutes w.  s. 

Calories  per  watt-second 

Calories  per  kilowatt-hour 

Cost  of  current  per  kilowatt  hour c. 


EFFICIENCY  TEST  OF  AN  ELECTRIC  MOTOR  21 

Discussion.  Explain  any  way  in  which  heat  generated  by 
the  lamp  may  escape  without  being  measured  in  this  experiment. 

Conclusion.  At  the  price  of  .  .  .  ff  per  kilowatt-hour,  the  cost 
of  raising  4  liters  of  water  from  15  to  100°  C.  will  be  .  .  .£ 
if  an  electric  heater  of  the  same  efficiency  as  the  lamp  is  employed. 

POWER 

A  series  of  exercises  designed  to  emphasize  the  difference 
between  work  and  power  as  measured  in  electrical  units  (e.  g., 
watt-hour  and  kilowatt)  is  commercially  of  primary  importance. 

In  "  A  High  School  Course  in  Physics/7  by  Gorton,*  will  be 
found  the  following  terse  and  lucid  definition: 

"  POWER  OF  AN  ELECTRIC  CURRENT.  Since  power  refers  to 
the  rate  at  which  work  is  done  or  energy  expended,  it  may  be 
found  by  simply  dividing  the  total  energy  expended  by  the  time. 
In  an  electric  circuit,  therefore,  the  power  is  measured  by  the 
product  of  the  potential  difference  and  the  current  strength;  or, 
Power  (watts)  =  volts  X  amperes. ' ' 


EXPERIMENT  VII 
EFFICIENCY  TEST   OF  AN  ELECTRIC   MOTOR 

There  is  a  rapid  increase  in  the  number  of  school  laboratories 
which  have  a  small  electric  motor.  These  motors  are  usually 
of  recent  design,  and  of  \  horse-power  or  over  in  rating. 

The  student  who  uses  motors  of  this  type  is  usually  impressed 
by  the  fact  that  he  is  working  with  real  commercial  quantities  of 
energy  and  with  actual  life-size  machines. 

A  determination  of  the  brake  horse-power  and  efficiency  of 
such  a  motor  is  considered  an  essential  part  of  the  laboratory 
course  in  many  of  our  high  schools.  Such  a  test  is  very  well 
described  in  "  Physics,"  by  Mann  and  Twiss,  as  follows: 

"  As  with  simple  machines,  water  motors,  and  steam  engines, 
the  most  important  thing  about  electric  machines  is  the  efficiency. 

"  Suppose  that  we  have  bought  a  110-volt  motor  that  is  built 
to  develop  two  horse-power,  and  we  wish  to  test  its  power  and 
efficiency  in  order  to  see  whether  it  does  what  is  claimed  for  it. 

*  D.  Appleton  &  Company,  Publishers.     New  York  and  Chicago. 


22 


ELEMENTARY, ELECTRICAL   TESTING 


The  voltmeter  F,  whose  terminals  are  attached  to  those  of  the 
motor  (Fig.  11),  measures  the  D  P  at  the  motor.  It  should  read 
110  volts.  The  ammeter  A,  placed  in  the  power  circuit  in  series 
with  the  machine,  measures  the  number  of  amperes  flowing 
through  the  motor.  Suppose  it  reads  16  amperes.  Then  the 
power  supplied  to  the  motor  (power  in)  is 

110  (volts)  X 16  (amperes)  =  1760  watts. 

"  A  brake  is  applied  to  the  axle  of  the  motor  and  the  readings 
made. 


LIN£ 


FIG.  11. 


"  Let  us  suppose  these  readings  to  be  as  follows: 

"  Circumference  of  brake  wheel,  2  feet;  revolutions  per  minute, 

360;  pull  on  brake,  82.5  pounds.     Then  the  power  obtained  from 

the  motor  is: 

82.5(lbs.)  X2(ft.)  X360(r .p.m.)  =59,400  ft.-lbs.  per  min. 

"  Dividing  this  by  33,000  ft.-lbs.  per  min.  to  reduce  it  to 
horse-power,  we  get  1.8  horse-power. 

"  So  1760  watts  of  electric  power  was  supplied  to  the  motor 
to  make  it  do  work  at  the  rate  of  1.8  horse-power.  This  result 
enables  us  to  compare  the  efficiency  of  this  motor  with  that  of 


DETERMINATION  OF  LOW  RESISTANCES  23 

others;  but  it  does  not  state  what  the  real  efficiency  of  this  motor 
is,  because  the  power  in  is  expressed  in  watts,  and  the  power  out 
is  expressed  in  horse-power."* 

RESISTANCE 

The  most  accessible  means  for  measuring  quantities  with 
commercial  accuracy  are  the  ones  most  frequently  ignored.  In 
the  measurement  of  resistance  it  is  perhaps  true  that  instructors 
neglect  the  voltmeter-ammeter  method  because  it  seems  so  ob- 
vious. Certainly  no  quicker  method  could  be  selected  than  that 
of  placing  an  ammeter  in  series,  and  then  connecting  a  voltmeter 
across  the  terminals  of  the  conductor  to  be  tested.  Nothing 
else  is  necessary  except  perhaps  a  rheostat  to  regulate  the  amount 
of  current. 

Every  student  knows  enough  of  algebra  and  of  Ohm's  law 
to  solve  for  the  resistance  when  he  has  taken  such  measurements. 
The  student  who  is  thus  early  asked  to  test  the  resistance  of  each 
phase  of  an  induction  motor,  the  fields  of  a  direct-current  motor, 
the  ballast  coils  in  an  arc  lamp,  or  any  other  electrical  device, 
starts  with  a  fair  appreciation  of  what  ordinary  commercial 
measurements  of  resistance  are  like. 

The  following  experiment  fully  describes  the  advantages  and 
limitations  of  this  method: 


EXPERIMENT  VIII 

THE    DETERMINATION    OF    LOW    RESISTANCES    BY    THE 
AMMETER  AND   VOLTMETER  METHOD 

Apparatus  Required:  Weston  ammeter;  Weston  millivolt- 
meter,  500  millivolt  range;  Weston  voltmeter,  3  and  15  volts; 
unknown  low  resistance;  storage  battery,  3  cells;  Weston  alloy 
or  carbon  rheostat. 

This  method  is  identical  in  principle  with  the  one  by  which 
the  fall  of  potential  along  a  conductor  is  determined.  Com- 
mercially, it  is  constantly  used  for  testing  the  resistances  of  bus- 
bars, dynamo  armatures,  shunts,  etc. 

*  See  also  Experiment  37,  Wauchope's  Manual  in  Physics.  Scott,  Foresman 
&  Co.  New  York  and  Chicago. 


24 


ELEMENTARY  ELECTRICAL  TESTING 


A  Weston  ammeter,  preferably  with  a  detachable  shunt 
having  a  standard  drop,  is  connected  in  series  with  the  resistance 
material  to  be  measured,  a  rheostat,  and  a  source  of  steady 
current. 

Current  is  regulated  until  a  suitable  ammeter  deflection  is 
obtained. 

If  the  resistance  of  the  material  to  be  tested  cannot  be  deter- 
mined approximately  by  calculation,  a  voltmeter  should  first  be 
connected  across  its  extremes  a  and  b  (see  Fig.  12)  and  a  pre- 


FIG.  12. 


liminary  test  made  to  learn  whether  the  potential  between  these 
points  is  too  great  to  permit  the  use  of  the  millivoltmeter.  If 
this  is  not  the  case,  the  millivoltmeter  is  substituted,  its  deflection 
noted,  and  the  resistance  between  a  and  b  is  found  by  the  formula 


/  being  the  current  and  E  the  e.m.f.  indicated  by  the  millivolt- 
meter. 

Strictly,  the  value  obtained  is  not  that  of  the  resistance 
material  only,  but  of  the  combined  resistance  of  the  material 
and  the  millivoltmeter  in  parallel. 

To  determine  the  resistance  accurately,  a  more  refined  method 
must  be  used. 

A  variation  of  the  above  method,  which  is  useful  for  measur- 
ing armature  resistance,  etc.,  results  from  the  use  of  a  single 
millivoltmeter,  and  a  known  low  resistor  approximately  equal 


DETERMINATION  OF  HIGH  RESISTANCES  25 

to  the  unknown  resistance,  in  place  of  the  shunt.  Then  by  first 
connecting  the  millivoltmeter  across  the  resistor,  the  current 
through  the  circuit  may  be  computed  by  Ohm's  law.  Next, 
by  placing  the  instrument  terminals  across  the  unknown  resist- 
ance, the  fall  of  potential  through  it  and  also  its  resistance  may 
be  then  determined.  With  steady  current,  this  method  is  as 
accurate  as  the  preceding  one,  but  since  it  is  assumed  that  the 
conditions  do  not  change  between  tests,  it  cannot  be  relied  upon 
with  fluctuating  current. 

INSULATION 

An  ideal  condition  of  affairs  would  be  to  have  a  perfect  con- 
ductor of  electricity  protected  by  a  perfect  insulator.  We  would 
then  always  secure  100  per  cent  efficiency,  irrespective  of  the 
length  of  cross-section  of  our  conductor.  But  unfortunately 
we  have  neither,  since  all  conductors  have  resistance  and  all 
insulators  are  imperfect  conductors.  The  result  is  that  there 
is  always  a  current  discharge  on  a  line.  This  discharge  or 
"  leakage  "  is  increased  by  the  use  of  poor  or  worn  insulating 
material  and  by  dampness. 

In  all  electric  light  and  power  stations,  leakage  is  regarded  as 
waste,  involving  a  direct  financial  loss.  Therefore,  it  is  of  great 
importance  to  be  able  to  "  keep  up  the  line  insulation  "  and  to 
understand  how  to  locate  leaks  or  "  grounds." 


EXPERIMENT  IX 

THE    DETERMINATION    OF    HIGH    RESISTANCES    BY   THE 
VOLTMETER  DEFLECTION   METHOD 

Apparatus  Required:  Weston  direct-current  voltmeter,  150- 
volt  range;  high  unknown  resistance;  direct-current  line  voltage, 
100  to  120  volts. 

On  a  direct-current  line,  in  order  to  successfully  determine 
insulation  resistance,  it  is  necessary  to  use  a  Weston  permanent 
magnet  movable-coil  type  of  high  resistance  voltmeter,  with  a 
uniformly  divided  scale.  The  resistance  of  the  instrument 
should  be  known. 


26 


ELEMENTARY  ELECTRICAL  TESTING 


The  normal  voltage  of  the  line  is  then  found  by  connecting 
the  voltmeter  across  the  line,  as  shown  in  Fig.  13,  and  the  deflec- 
tion noted. 


FIG.  13. 

One  binding  post  of  the  instrument  is  then  grounded  by  con- 
necting it  with  any  convenient  water,  gas  or  steam  pipe,  and 
the  other  binding  post  connected  directly  with  one  of  the  gener- 
ator terminals.  The  other  generator  terminal  is  connected  with 
the  line  to  be  tested  and  the  resultant  deflection  also  noted. 
See  Fig.  14. 


•=  6 


FIG.  14. 


The  resistance  of  the  line  (in  ohms)  is  then  found  by  the  for- 
mula : 

Y     SXR     7? 

X—gr-B. 


S  =  First  deflection  in  scale  divisions; 
S'  =  Second  deflection  in  scale  divisions; 
R  =  Instrument  resistance. 


THE  WHEATSTONE  BRIDGE 


27 


Example : 


£=120; 


'  =  20; 


=  15000; 


X= 


X  =  75000. 

The  insulation  resistance  of  a  line  or  cable  may  readily  be 
determined  in  this  manner  if  it  is  not  too  high. 

Sometimes,  when  the  insulation  is  very  poor,  a  fine  class- 
room illustration  can  be  made  by  grounding  the  line  through  an 
ammeter  in  series  with  an  incandescent  lamp,  as  shown  on 
Fig.  15. 


="  G 


FIG.  15. 


EXPERIMENT  X 
THE   WHEATSTONE  BRIDGE 

The  Wheatstone  bridge  remains  the  time-honored  instru- 
ment for  the  highly  accurate  determination  of  resistance.  In 
commercial  testing  laboratories,  it  is  regularly  used  for  routine 
and  special  work,  demanding  greater  precision  than  can  readily 
be  obtained  by  more  rapid  methods.  Its  use  is  also  common 
for  measurements  in  which  the  large  currents  or  voltages  required 
by  other  methods  are  either  unobtainable  or  objectionable. 
As  an  illustration,  reference  might  be  made  to  its  use  in  telegraph 
and  telephone  tests  for  faults  and  grounds.  The  Wheatstone 
bridge  is  operated  by  specially  trained  workers,  and  for  educa- 
tional purposes  it  should  not  be  classed  with  voltmeters,  ammeters, 
and  wattmeters  as  an  instrument  with  which  every  high-school 
graduate  should  be  familiar. 


28 


ELEMENTARY   ELECTRICAL  TESTING 


Many  teachers  question  the  wisdom  of  attempting  to  teach 
beginners  the  principle  and  operation  of  this  instrument,  as  much 
because  of  the  indifferent  success  which  so  frequently  results  as 
because  of  its  special  character.  This  discussion  is  intended  for 
those  teachers  who  feel  that  they  must  attempt  its  use.  The 
slide-wire  form  is  here  suggested,  not  only  because  it  is  already 
in  common  use,  but  also  because  it  lends  itself  to  a  close  corre- 
lation between  the  "  Fall  of  Potential "  exercises  which  should 
precede  it. 

Apparatus  Required:  Slide  wire  bridge;  Weston  student 
galvanometer;  Weston  ammeter;  resistance  box;  resistance 


FIG.  16. 


wire;  coil  of  unknown  resistance;  one  or  two  cells;  incandescent 
lamps. 

If  four  incandescent  lamps  are  arranged  in  multiple  series, 
as  shown  in  Fig.  16,  and  connected  with  the  line,  they  will  serve 
excellently  to  demonstrate  the  principle  of  the  parallelogram 
of  forces,  as  exemplified  by  the  Wheatstone  bridge.  Even  if 
the  rest  of  the  exercise  is  omitted,  this  part  should  be  given  as  a 
class-room  demonstration.  The  instrument  should  preferably 
be  a  zero  center  ammeter. 

Suitable  questions  would  be  as  follows: 

If  an  ammeter  is  connected  (as  shown),  what  will  happen? 

Will  it  indicate,  and  if  not,  why  not? 

If  one  of  the  lamps  is  removed,  what  will  be  the  result? 


THE  WHEATSTONE  BRIDGE 


29 


If  a  lamp  of  different  size  (resistance)  is  substituted,  will  it 
make  any  difference? 

If,  instead  of  the  lamps,  a  parallelogram  of  resistance  wire 
is  constructed  so  that  the  four  sides  have  the  same  resistance, 
and  a  battery  and  galvanometer  are  connected,  as  shown  in  Fig. 
17,  the  current  will  split  where  A  and  B  unite,  pass  through 
A  and  X  and  also  through  B  and  R,  reuniting  where  R  and  X 
join.  The  four  "  arms  "  of  the  bridge  being  alike,  no  current 
will  flow  through  the  galvanometer  when  connected  as  shown. 
In  other  words,  the  potential  at  the  junction  of  AX  and  BR 


FIG.  17. 


will  be  the  same.     The  battery  and  galvanometer  may  then 
be  interchanged  without  affecting  the  balance. 

To  construct  a  practical  bridge,  it  is  only  necessary  to  sub- 
stitute for  A ,  B}  X ,  and  R  conductors  of  large  area  and  negligible 
resistance  with  suitable  gaps  for  the  insertion  of  resistance  coils 
and  the  material  to  be  measured.  See  Fig.  18.  If  then,  A  and 
B  are  each  100  ohms  and  R  is  also  100  ohms,  a  balance  is  secured 
by  adjusting  X,  and  when  no  current  flows  through  the  gal- 
vanometer the  resistance  of  X  will  be  100  ohms.  If  the  resistance 
of  X  is  to  be  determined,  R  is  made  adjustable  and  is  varied 
until  it  equals  X.  The  resistance  of  R  and  X,  if  equal  to  each 
other,  may  differ  greatly  from  A  and  /?,  and  an  equilibrium 
be  established,  provided  A  and  B  equal  each  other.  Finally, 
A,  B,  R,  and  X  may  all  differ  in  resistance  and  a  balance  be 


30 


ELEMENTARY  ELECTRICAL  TESTING 


secured,   provided   a   proportionality   exists   in  their   respective 
resistances  —  for  instance,  when 


A  =  10,     £  =  100, 


and 


350. 


These  conditions  obtain  when  an  uneven  bridge  or  ratio  is  used 
in  making  measurements. 


FIG.  18. 


EXPERIMENT   XI 
THE   SLIDE   WIRE   BRIDGE 

In  the  slide-wire  form  of  bridge  A  and  B  consist  of  a  single 
wire  of  uniform  resistance.  Contact  may  be  made  at  any  point 
on  this  wire,  and  a  scale  is  provided  to  determine  the  position 
of  contact.  See  Fig.  19.  The  galvanometer  is  preferably  con- 
nected as  shown,  an  unknown  resistance  is  inserted  at  X  and  a 
box  of  coils  at  R.  The  slide- wire  contact  is  at  first  placed  cen- 
trally, and  an  approximate  balance  secured  by  varying  R.  A 
place  is  then  found  on  the  slide  wire  where  perfect  balance  is 
secured. 

Since  the  scale  is  divided  into  a  thousand  parts,  the  resistance 
of  X  can  be  closely  determined  by  simple  proportion. 


EFFECT  OF  TEMPERATURE 


31 


EXPERIMENT   XII 

THE  EFFECT  OF  TEMPERATURE  ON  THE  RESISTANCE  OF 
A  LAMP  FILAMENT* 

Apparatus:  110- volt  direct  current;  series  lamp  resistance  or 
any  suitable  adjustable  resistance;  switch  with  6-ampere  fuses; 
ammeter;  voltmeter;  lamps  to  be  tested.  (These  should  be 
of  several  types.) 

The  difference  between  the  resistance  of  an  incandescent  lamp 
filament  when  the  switch  to  power  is  first  closed  and  the  resistance 


FIG.  19. 

of  the  same  filament  an  instant  later  when  the  filament  has  been 
raised  to  its  normal  operating  temperature,  is  a  matter  of  con- 
siderable commercial  importance.  With  the  old  type  carbon 
filaments,  where  the  temperature  effect  is  to  give  a  smaller 
initial  current,  the  phenomenon  could  be  safely  allowed  to  adjust 
itself.  With  the  newer  metallic  filament  lamps  in  which  the  first 
rush  of  current  may  be  much  greater  than  the  normal  operating 
current,  some  protection  to  the  line,  when  the  power  switch  is 
first  closed,  is  frequently  necessary  where  large  groups  of  lamps 
are  being  supplied.  The  character  and  extent  of  this  temperature 

*From    "The    Loose  Leaf    Laboratory   Manual,"    published  by  John 
Wiley  &  Sons,  Inc. 


32 


ELEMENTARY  ELECTRICAL  TESTING 


effect  varies  with  various  types  of  lamp  filaments  will  be  apparent 
from  the  following  study. 

Method.  Connect  your  apparatus  with  the  110-volt  circuit 
as  shown  in  the  diagram,  using  first  a  carbon  filament  test  lamp. 
(The  voltmeter  is  here  connected  around  the  ammeter  so  that 
the  latter  may  read  the  true  current  through  the  lamp.)  Put 
a  fuse  in  the  line  to  protect  the  ammeter.  See  Fig.  20. 

Data  and  Results.  Take  readings  of  both  the  ammeter  and 
voltmeter  with  all  the  lamps  in  the  lamp  board  in  series  with  the 
test  lamp.  Then  cut  out  one  lamp  at  a  time,  and  take  readings 
of  the  voltmeter  and  the  ammeter  for  each  step,  until  the  last 


EFFECT  OF  TEMPERATURE  ON  THE 
RESISTANCE  OF  AN  INCANDESCENT  LAMP 


FIG.  20. 


reading  is  for  the  test  lamp  alone.  The  ammeter  and  the  voltmeter 
should  be  read  simultaneously. 

Compute  the  resistance  of  the  test  lamp  for  each  current. 

You  have  no  actual  measurement  of  temperature  here,  but 
as  the  current  increases  the  temperature  of  course  rises — very 
perceptibly  for  the  last  two  or  three  readings. 

Plot  a  curve  from  your  data,  using  the  resistances  of  the 
lamp  as  ordinates,  the  current  as  abscissae. 

What  conclusion  may  be  drawn  regarding  the  effect  of  increase 
of  temperature  on  the  resistance  of  the  filament? 

Test  in  the  same  way  lamps  having  a  treated  carbon  filament 
and  lamps  having  a  metal  filament  as  in  the  tantalum  and  tungsten 
lamps.  Plot  curves  for  each  and  compare  the  filaments  as  to 
their  change  in  resistance  with  rise  in  temperature. 


EFFECT  OF   TEMPERATURE  33 

Addenda.  Connect  the  voltmeter  across  the  test  lamp  only, 
and  note  the  ammeter  reading.  Compare  this  with  the  ammeter 
reading  taken  with  the  voltmeter  connected  across  both  the 
lamp  and  ammeter.  Which  ammeter  reading  gives  the  true 
current  through  the  lamp,  and  why?  Which  voltmeter  reading 
gives  the  true  voltage  across  the  lamp,  and  why?  When  would 
it  be  best  to  connect  the  voltmeter  across  both  the  ammeter 
and  test  piece?  When  across  the  test  piece  only? 

Give  detailed  reasons  for  your  answer.  (See  "  Elements  of 
Electricity,"  p.  413.) 

PROBLEMS:  1.  Measure  on  a  Wheatstone  bridge  the  resistance  .of  the 
tungsten  filament  lamp  at  room  temperature  (20°  C.).  Using  the  tempera- 
ture coefficient  of  tungsten  as  .0051,  and  the  resistance  found  for  the  lamp 
in  the  above  experiment  when  on  its  rated  voltage,  compute  the  temperature 
of  the  filament  when  used  on  its  rated  voltage. 

2.  In  an  experiment  like  the  above  the  voltmeter  was  connected  across 
the  test  lamp  only.  If  the  resistance  of  the  voltmeter  is  16,500  ohms,  the 
voltmeter  reading  is  110  volts,  and  the  ammeter  reading  is  .285  amp.,  what 
is  the  true  current  through  the  lamp  filament? 


THE   WESTON   DIRECT-CURRENT  AMMETER 

For  pedagogic  purposes  the  Weston  ammeter  with  detachable 
shunts  has  marked  advantages  over  the  instrument  with  a  self- 
contained  shunt.  The  instructor  may  call  attention  to  the 
fact  that  a  "  shunt  "  is  really  a  part  of  the  main  conductor  carry- 
ing the  current  and  that  only  a  small  fraction  of  the  total  cur- 
rent passes  through  the  movable  system  of  the  instrument. 

A  clear  conception  of  the  operation  of  a  Weston  ammeter 
may  be  obtained  by  means  of  a  water-main  analogy.  The  water 
main  or  trunk  A  through  which  water  is  flowing  under  pressure 
as  in  city  water  mains  (Fig.  21),  may  be  fitly  compared  with  the 
conductor  or  bus-bar  F  (Fig.  22).  The  section  B  of  reduced 
diameter  corresponds  with  the  shunt  E,  and  the  small  pipes  C 
may  be  compared  with  the  leads  (7.  It  is  obvious  that,  since  the 
pipes  C  are  of  smaller  dimensions  than  B,  less  water  will  flow 
through  them;  and,  provided  that  the  proportion  between 
B  and  C  is  fixed  and  constant,  the  quantity  of  water  flowing 
through  the  meter  D  will  depend  upon  the  total  quantity  flowing 
through  B,  and  hence  D  may  be  calibrated  to  indicate  the  total 


34 


ELEMENTARY  ELECTRICAL  TESTING 


flow  of  water  instead  of  merely  indicating  the  quantity  passing 
through  C. 

On  the  same  principle  the  scale  of  the  instrument  H  may  be 
figured  to  indicate  any  required  amperage;    but,  although  the 


FIG.  21. 


total  current  flowing  through  F  may  be  20,000  amperes  or  more, 
the  sensitivity  of  the  movement  of  H  to  current  is  so  great  that 
only  about  .3/100  ampere  is  required  to  produce  a  full  scale 
deflection,  and  under  proper  conditions  no  current  in  excess  of 


Fio.  22. 


this  amount  will  ever  pass  through  H.  The  conductors  G, 
together  with  the  movable  coil  and  resistors  inside  of  H,  are  so 
proportioned  that  the  flow  of  current  through  H  is  limited  to  the 
proper  amount.  But,  nevertheless,  instead  of  merely  indicating 


THE  STUDY  OF  AN  AMMETER  35 

the  amount  of  current  passing  through  its  movement,  H  may 
be  calibrated  to  correctly  indicate  the  total  current  flowing 
through  FEF. 

Finally,  the  quantity  of  current  which  will  flow  through  G 
will  depend  upon  the  resistance  of  the  shunt  E;  therefore,  since 
shunts  of  different  resistance  and  current  capacity  may  be  used 
in  place  of  E,  the  instrument  H  may  be  used  for  an  unlimited 
number  of  ampere  ranges.* 


EXPERIMENT   XIII 
THE   STUDY   OF  AN   AMMETER  f 

Apparatus:   Ammeters  of  several  types. 

Ammeters  may  be  divided  into  two  classes:  (1)  Thermal — 
in  which  the  movement  of  the  index  is  secured  through  the  change 
in  length  of  a  wire  when  heated  by  the  current  passed  through 
it.  The  heat  generated,  and  therefore  the  change  in  length, 
is  here  proportional  to  the  square  of  the  current;  and 

(2)  Electromagnetic — in  which  the  motion  is  due  to  the 
magnetic  field  produced  when  current  is  sent  through  the  coils 
or  coil  of  the  instrument.  Electromagnetic  ammeters  may  be 
of  three  types:  (a)  Solenoidal;  (6)  permanent  magnet;  (c) 
elect rody  namometer . 

Data  and  Results.  Examine  instruments  of  several  of  the 
above  types  and  report  upon  the  following  features  for  each : 

1 .  The  type  of  instrument. 

2.  The  scale:    range  and  graduation.     Divisions — uniform  or 

varying  width.     Why? 

*  A  shunt  in  electrical  parlance  was  formerly  defined  as  a  resistor  connected 
in  multiple  with  an  indicating  instrument  carrying  the  main  current  to 
reduce  the  current  flowing  through  the  latter;  but,  since  the  introduction 
of  Weston  instruments,  "shunt"  has  become  the  trade  name  for  a  constant 
resistor  of  special  form,  having  dimensions  which  will  permit  its  practical 
use  in  series  with  a  conductor  carrying  a  current,  independent  of  the  current 
capacity  of  its  indicating  instrument. 

t  From  "  The  Loose  Leaf  Laboratory  Manual."     John  Wiley  &  Sons, 
Inc. 


36  ELEMENTARY  ELECTRICAL  TESTING 

3.  The  construction: 

(a)   Fixed    parts;     nature,     material,     construction    and 

arrangement. 
(6)  Moving    parts;     construction,    bearings;     control    by 

which  they  are  returned  to  zero;   damping. 
(c)  Shunt. 

4.  The  connections  by  which  the  current  enters  and  leaves 

the  instrument.     The  circuit  from  bind  post  to  bind  post. 

5.  The  resistance  of  each  coil  and  of  each  shunt  separately. 

The   resistance   of   the   instrument   from   terminal   to 
terminal. 

Explain  the  construction  and  arrangement  of  each  essential 
part  by  simple  diagrams,  and  show  by  a  simple  diagram  how 
the  parts  are  assembled  in  the  complete  instrument.  Show  the 
direction  of  current  through  each  instrument  and  explain  the 
reason  for  the  deflection  of  the  moving  parts  by  the  current. 
State  in  each  case  whether  the  instrument  is  suited  for  the  mea- 
surement of  direct  or  alternating  current  and  why. 

If  the  instrument  is  suited  for  both  direct  and  alternating 
current,  will  the  same  calibration  do  for  both  kinds  of  power? 
Explain  the  reason  for  your  answer. 

By  means  of  a  millivoltmeter  measure  the  millivolt  drop 
across  each  instrument  on  full  scale  reading  and  compare  this 
with  the  computed  value.  Account  for  any  difference. 

Show  by  diagrams  the  proper  directions  of  the  instrument 
to  a  line  to  measure  the  current  taken  by  a  motor. 

PROBLEMS.  The  full  scale  reading  of  a  millivoltmeter  is  100  millivolts. 
The  resistance  of  the  moving  coil  is  5  ohms. 

(a)  What  resistance  must  a  shunt  be  in  order  to  be  used  with  this  milli- 
voltmeter to  have  the  scale  read  amperes? 

(6)  What  must  the  resistance  of  a  shunt  be  in  order  that  the  full  scale 
reading  may  indicate  10  amperes? 

SUGGESTIONS   FOR   FURTHER   STUDY 

If  the  "drop"  of  a  West  on  station  shunt  of  specified  ampere 
capacity  is  50  millivolts,  what  will  its  resistance  be  between  its 
potential  terminals? 


THE  STUDY  OF  AN  AMMETER  37 

Detach  the  shunt  of  a  Weston  ammeter  with  removable 
shunts,  and  measure  the  resistance  of  the  instrument  by  means 
of  a  bridge. 

Note:  Minimum  current  should  be  used,  and  the  battery 
key  kept  depressed  to  prevent  the  disturbing  effect  of  currents 
induced  by  the  motion  of  the  movable  coil. 

If  the  drop  of  a  shunt  is  50  millivolts,  the  instrument  for 
which  it  is  intended  must  necessarily  give  a  full  scale  deflection 
with  50  millivolts.  What  then  will  be  the  amount  of  current 
required  to  give  a  full  scale  deflection  when  the  instrument  is 
used  without  a  shunt? 

Explain  what  the  effect  would  be  if  the  leads  from  the  shunt 
to  the  instrument  were  altered  in  length  or  resistance. 

Determine  the  power  (in  watts)  consumed  by  the  instrument 
with  shunt  when  in  circuit  with  full  scale  deflection. 


THE   WESTON   DIRECT-CURRENT  VOLTMETER 

The  Weston  direct-current  movable  coil  voltmeter  consists 
essentially  of  a  light  rectangular  coil  of  copper  wire  usually 
wound  upon  an  aluminum  frame,  pivoted  in  jeweled  bearings 
and  mounted  to  rotate  in  an  annular  space  between  the  soft  iron 
core  and  the  specially  formed  pole  pieces  of  a  permanent  magnet. 
A  light  tubular  pointer  is  rigidly  attached  to  the  coil  and  moves 
over  a  calibrated  scale. 

The  current  is  led  into  and  from  the  coil  by  means  of  two  spiral 
springs,  which  serve  also  to  control  its  movement.  This  move- 
ment is  due  to  the  dynamic  action  between  the  current  flowing 
through  the  coil  and  the  magnetic  field  of  the  permanent  magnet. 
(See  Fig.  23.)  The  pointer  becomes  stationary  and  the  coil  attains 
a  position  of  equilibrium  when  the  opposing  forces  of  the  springs 
equal  the  force  caused  by  the  rotary  tendency  of  the  coil.  Since 
the  magnetic  field  is  uniform  and  the  torsion  of  the  springs 
proportional  to  the  deflection,  the  scale  divisions  are  practically 
uniform. 

The  well-known  aperiodic  or  "  dead-beat  "  quality  of  Weston 
instruments  is  produced  in  this  type  by  foucault  currents  gen- 
erated in  the  aluminum  frame  when  rotating  through  the  mag- 
netic field.  These  Foucault  currents  have  a  sufficient  influence 


38  ELEMENTARY  ELECTRICAL  TESTING 

on  the  movement  of  the  coil  to  cause  it  to  come  to  rest  almost 
instantly  and  without  friction. 

Ball,  Hauptman  and  Bateman,  when  dealing  with  the  subject 
of  Potential  Differences,*  make  the  following  statement,  which 
we  quote  not  only  because  it  is  well  expressed,  but  also  since  it 
serves  excellently  to  describe  the  proper  conditions  under  which 
voltmeters  may  be  uesd : 

"  It  has  already  been  stated  and  shown  in  the  previous  experi- 
ments that,  when  certain  electrical  generators  are  employed,  the 
P.D.'s  between  bodies  charged  from  such  generators  may  remain 
unchanged  even  when  these  bodies  are  connected  by  a  conductor. 
This  effect  may,  under  proper  conditions,  be  made  the  basis  of 
measurement  of  P.D.  The  instruments  generally  employed  for 
the  commercial  measurement  of  P.D.  are  based  upon  this  principle. 
It  must  be  noted,  however,  that  the  use  of  such  instruments 
is  admissible  only  when  the  current  which  passes  through  them 
does  not  alter  the  P.D.  between  the  points  to  which  they  are 
connected." 

If,  therefore,  an  instrument  is  used  to  measure  this  P.D.,  it 
should,  to  perform  its  functions  properly,  be  so  constructed  that 
it  is  extremely  sensitive  to  current. 

Weston  voltmeters  fulfill  these  conditions.  It  is  true  that 
they  are  conductors  and  require  a  small  current  to  render  them 
operative,  and  that  there  are  conditions  under  which  they  would 
not  indicate  correctly.  For  instance,  they  could  not  be  used 
with  marked  success  to  determine  the  static  charge  of  a  Holtz 
machine  or  a  Ley  den  jar.  Neither  would  they  correctly  indicate 
the  potential  of  a  Zamboni  dry  pile,f  nor  should  they  be  used  to 
directly  test  the  e.m.f.  of  a  Weston  standard  cell. 

But  since  Weston  voltmeters  require  only  about  1/100  ampere 
with  full  scale  deflection,  there  will  be  no  appreciable  fall  in 
potential  when  they  are  connected  across  any  commercial  source 
of  current,  ranging  from  a  light  and  power  plant  to  a  dry  cell. 

An  exercise  similar  to  the  following  one,-  on  the  "  Study  of 
a  Voltmeter,"  should  in  our  opinion  form  part  of -every  labora- 
tory course  in  physics. 

*  Experiment  No.  107,  Laboratory  Experiments,  1913. 
t  Deschanel's  Natural  Philosophy,  1870. 


THE  STUDY  OF   A  VOLTMETER  39 

EXPERIMENT  XIV 
THE  STUDY  OF  A  VOLTMETER* 

Apparatus  Required:    Voltmeters  of  several  types. 

Voltmeters  may  be  of  the  types  suggested  in  preceding 
experiment,  or  may  be  electrostatic. 

Examine  instruments  of  several  types  and  report  upon  the 
following  for  each : 

1.  The  type  of  instrument. 

2.  The  scale:    range  and  graduation.     Divisions — uniform  or 

varying  width.     Why? 

3.  The  construction: 

(a)  Fixed    parts:    nature,    material,    construction    and 

arrangement. 
(6)  Moving  parts:    construction;    bearings;    control  by 

which  returned  to  zero;   damping, 
(c)  Series  coils.     Double-scale  instruments. 

4.  The  connections  by  which  current  enters  and  leaves  the 

instrument.     The  circuit  from  binding  post  to  binding 
post. 

5.  The  resistance  of  the  instrument.     Resistance  per  volt. 

Explain  the  construction  and  arrangement  of  each  essential 
part  by  a  separate  diagram,  and  show  by  a  simple  diagram  how 
the  parts  are  assembled  in  the  complete  instrument.  Show  the 
direction  of  current  through  the  instrument  in  each  and  give  the 
reason  for  the  deflection  of  the  movable  parts.  Why  does  the 
reading  indicate  the  difference  of  potential  at  the  terminals? 
State  in  each  case  whether  the  instrument  is  suited  to  the  meas- 
urement of  direct  or  alternating  voltages  and  why.f 

*  From  "  Physical  Laboratory  Notes,"  by  J.  M.  Jameson.* 
t  See  also  "Electrical  Instruments  and  Testing,"  (Chapter  IV),  by  Schnei- 
der and  Hargrave,  published  by  Spon  &  Chamberlain,  N.  Y.,  also  "Lessons 
in   Practical    Electricity,"  Lesson   XVIII,  Swoope,  published   by  D.  Van 
Nostrand  Co.,  N.  Y. 


40 


ELEMENTARY  ELECTRICAL  TESTING 


Suggestions  for  Further  Study.  Note  or  measure  the  resist- 
ance of  one  or  more  ranges  of  a  Weston  voltmeter  of  the  movable 
coil  type. 

Determine  the  current  actually  flowing  through  the  instru- 
ment with  full  scale  deflection. 

Determine  the  power  (in  watts)  required  to  produce  a  full 
scale  deflection  with  different  ranges. 


FIG.  23. 


Explain  why  a  voltmeter  the  resistance  of  which  is  high  (in 
ohms  per  volt)  is  preferable  to  one  of  low  resistance. 


Explain  the  construction  of  a 
ment  and  field. 


Weston   "  soft-iron  "   move- 


EXPERIMENT  XV 
THE  ACTION   OF  A  SIMPLE   CELL 

Since  generators  and  storage  cells  have  taken  the  place  of  the 
primary  batteries  once  largely  used  for  the  commercial  produc- 
tion of  current,  the  exercise  which  follows  may  be  rated  by  many 
instructors  as  too  abstract  to  deserve  a  place  in  a  practical  physics 


THE  ACTION  OF  A  SIMME  CELL' 


41 


course.  However,  nearly  every  modern  text  book  still  retains 
it,  and  it  remains  for  the  instructor  to  decide  whether  it  suffices 
to  merely  refer  to  Volta's  discoveries,  or  emphasize  their  signifi- 
cance by  means  of  an  exercise  which  enables  the  student  to  pro- 
duce current  by  chemical  action. 

Apparatus  Required:  Low  range,  direct-current,  5-ampere 
Weston  ammeter;  low  range,  direct-current,  1.5- or  3-volt  Weston 
voltmeter;  strips  of  sheet  zinc,  2X6  inches;  strips  of  sheet 
copper,  2X6  inches;  glass  jar,  about  5  inches  in  diameter  and  4 
inches  deep;  ten  per  cent  sulphuric  acid  solution;  mercury; 
copper  wire. 


FIG.  24. 


FIG.  25. 


Copper  leads  are  to  be  soldered  to  the  plates,  the  latter  being 
bent  so  that  they  may  hang  in  the  solution  from  the  edge  of  the 
jar. 

The  voltmeter  is  first  connected  and  deflection  noted,  which 
will  be  about  1  volt.  See  Fig.  24. 

The  ammeter  is  then  substituted  and  deflection  also  noted. 
Current  when  circuit  is  first  completed  will  be  about  1  ampere; 
but  since  polarization  rapidly  sets  in,  the  current  at  once  decreases. 
Why? 

Note  the  large  number  of  hydrogen  bubbles  passing  to  the 
surface  of  the  liquid  from  the  zinc  plate.  Why? 

Substitute  an  amalgamated  zinc  plate.     Note  that  no  hydro- 


42  ELEMENTARY  ELECTRICAL  TESTING 

gen  forms  except  when  the  circuit  is  closed,  and  that  the  current 
is  comparatively  constant.  Why? 

With  the  ammeter  in  circuit,  decrease  the  distance  between 
the  zinc  and  copper  poles,  and  note  the  increase  in  current.  Why? 
See  Fig.  25. 

Disconnect  the  ammeter  and  try  the  above  experiment  with 
the  voltmeter.  Note  that  there  is  no  appreciable  difference 
in  the  e.m.f.  Why? 

With  the  plates  hanging  from  the  edge,  connect  both  instru- 
ments successively,  and  note  their  deflections.  Determine  the 
resistance  of  the  circuit  (cell,  ammeter  and  leads)  by  the  formula 


R  =  resistance  of  circuit;        E  =  e.m.f.;        /  =  current. 

When  both  instruments  are  connected  at  the  same  time,  the 
indicated  e.m.f.  of  the  voltmeter  is  much  less  than  it  is  when  the 
ammeter  is  not  in  parallel.  Why? 

NOTE.  In  making  the  above  experiments  the  student  may  be 
permitted  to  connect  the  ammeter  as  stated,  because  the 
resistance  of  the  cell  is  sufficiently  high  to  limit  the  flow  of  current 
to  a  safe  amount.  He  should,  however,  be  cautioned  about 
using  an  ammeter  in  this  manner  when  the  resistance  of  a  cell 
is  likely  to  be  low,  and  consequently  its  current  is  large.  For 
instance,  an  ordinary  dry  cell,  when  new,  will  have  so  low  a 
resistance  that  it  will  sometimes  give  as  much  as  30  amperes 
for  a  short  time  when  short-circuited  through  a  Weston  ammeter. 
Another  point  of  importance  is  that  the  resistance  of  a  Weston 
voltmeter  is  so  high  that  a  small  decrease  in  the  resistance  of 
the  circuit,  of  which  it  forms  a  part,  caused  by  bringing  the  plates 
closer  together,  will  have  too  slight  an  effect  to  materially  change 
the  total  current  flowing  under  the  conditions.* 

*  See  also  Gorton's  "High  School  Physics,"  Chapter  XVIII;  Fuller  and 
Brownlee's  "Laboratory  Exercises,"  Experiment  68;  "Physics,"  Mann  and 
Twiss,  page  58;  and  "Physics  Laboratory  Manual,"  by  Cavanagh,  West- 
cott  and  Twining.  Ginn  Company,  Publishers,  New  York. 


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